The present disclosure generally relates to data storage, and more particularly, to an optical head that couples radiation energy or other signals.
A typical optical storage system uses an optical head to focus a monochromatic optical beam to a small spot on a recording layer for reading and writing. The optical head has an optical interfacing surface that couples optical radiation to and from the storage medium. The interfacing surface may be an optical surface of a lens, an optical flat, or a transparent mesa formed as part of a lens in the optical head. The spacing between the interfacing surface and the storage medium is a critical device parameter that can determine the storage capacity and affect the operation of a storage system.
The spacing may be greater than one wavelength so that the optical energy is coupled between the optical head and the medium by light propagation. An optical head in such a configuration is in a xe2x80x9cfar-fieldxe2x80x9d optical configuration. The numerical aperture of the head in a far-field configuration is less than unity. The minimum beam spot size projected on the medium by the head is limited by the diffraction of light to a limit on the order of one half wavelength. Accordingly, the areal density of such an optical storage device is limited by this minimum diffraction-limited spot size.
The areal density in optical storage can be increased beyond the diffraction limit by using an optical storage device in a xe2x80x9cnear-fieldxe2x80x9d configuration where the interfacing surface of the optical head is spaced from the medium by a distance on the order of or less than one wavelength. The optical energy can be coupled between the optical head and the medium by evanescent coupling, with or without ordinary light propagation. The numerical aperture of the optical head in such a near-field configuration can be greater than unity. This is not possible in a far-field configuration. Hence, a near-field optical storage system can produce a focused beam spot size less than one half wavelength to achieve a high areal storage density beyond the capability of many far-field systems.
The spacing between the optical head and the storage medium in a near-field configuration can be maintained by an air bearing. In a near-field optical disk drive, the spinning motion of the disk relative to the optical head can generate a lifting force on an air-bearing surface formed in the optical head. This force can be sufficient to xe2x80x9csuspendxe2x80x9d the head over the surface of the disk at a desired distance less than one wavelength, e.g., in a range from about 10 nm to about 200 nm in some implementations.
The spacing should be maintained at a desired constant spacing within a predetermined tolerance range in order to achieve a repeatable and optimal performance. Any variation or defect in the geometry and surface quality of the air-bearing surface of the head can affect this spacing. Hence, the tolerances of the geometry and surface quality of the air-bearing surface of the head are usually strenuous and often cause a low yield in manufacturing the head.
The present disclosure provides a mechanism in an optical head that allows a controlled adjustment of the spacing between the interfacing surface and the storage medium. This adjustment can be used to optimize the performance of the head by maintaining the spacing at a desired value. In addition, the spacing between the interfacing surface and the medium may be different from the distance between the air bearing surface and the medium. It may be desirable to adjust these two distances separately. The adjustment can also compensate for variations or defects in the geometry and surface quality of the interfacing surface to allow use of a head that would otherwise be unusable. This effectively improves the yield in fabrication of the head.
One embodiment of an optical disk drive includes an optical head, a detection unit, an electrical heater, and a control circuit. The optical head includes an optically transparent interfacing surface to couple optical signals. The detection unit is configured to receive and detect a spacing-indicating signal from the optical head that indicates a spacing between the interfacing surface and the disk. The heater is disposed in the optical head to receive an electric current to supply heat so as to thermally shift the interfacing surface relative to the disk. The control circuit receives spacing information from the detection unit and controls the electric current to maintain the spacing at a desired value.
The optical head may include a transparent mesa structure having a mesa surface to effect the interfacing surface. The spacing-indicating signal may include a reflected optical signal from the optical head that has a dependence on said spacing so that the actual spacing can be determined. The spacing dependence of the reflected optical signal may be calibrated with reference to a contact point between the disk and the head, e.g., by using an optical distortion caused by mass transfer from the disk to the interfacing surface or an acoustic wave generated when the head comes into contact with the disk.
A method for operating an optical disk drive to maintain a desired head-disk spacing is also provided to maintain the spacing between the optical head and the disk. Heat is supplied to the optical head to control a position of the interfacing surface by thermal expansion. According to one embodiment, the spacing control includes: (1) detecting a reflected optical signal from the optical head to determine the actual spacing between the interfacing surface and the disk, (2) determining a difference between the actual spacing and a desired spacing; and (3) adjusting an amount of heat to the optical head to reduce the difference.
These and other aspects and associated advantages will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.